Make the request blocking mecanism in IS flux an option

Right now when an update of an inter-synthese flux is made on the client side, all HTTP requests are blocked until the import is finished. This is true only if the import has been launched automatically, not manually.

The problem here is that import block on each other because of the way MySQL calls are handled. When a BDSI import is running for example, and an IS import is made, all requests will block until both the BDSI and the IS import are done, which can be between 4 to 40 seconds. This makes the web interfaces unresponsive.

To test :

• Run a big IS flux import automatically and check that the requests can't be made while it's running
• Change the option and do it again, this time the requests can be made
• Try to do two imports IS flux at the same time and check they wait on each other